This application proposes to study the stabilizing structures of the distal radio-ulnar joint (DRUJ). The overall goal of the work is to increase understanding of the distal radio-ulnar joint and improve surgical treatments for distal radio-ulnar joint subluxation/dislocation. The proposal involves three studies. The first study examines the relative contribution of the volar radio-ulnar ligament, the dorsal radio-ulnar ligament, the ulnocarpal complex, the extensor carpi ulnaris subsheath and the interosseous membrane, on the stability of the distal radio-ulnar joint. These contributions will be evaluated through displacement-controlled stiffness testing. The three hypotheses related to this Aim include: 1a) the volar radio-ulnar ligament stabilizes the DRUJ against volar displacement of the radius relative to the ulna in all positions of forearm rotation; 1b) the dorsal radio-ulnar ligament stabilizes the DRUJ against lateral displacement of the radius relative to the ulna in all positions of forearm rotation; and 1c) the interosseous membrane stabilizes the DRUJ against proximal-distal displacement of the radius relative to the ulna in all positions of forearm rotation. The joint will be loaded in one of the proximal-distal, medial-lateral, and dorsal-volar directions at a time, to evaluate contributions to stability in different displacement modes. Stiffness will be repeatedly measured after sequential sectioning, to determine the relative contribution of each passive structural restraint. In the second study, the effect of two dynamic stabilizers, the extensor carpi ulnaris and the pronator quadratus (deep and superficial heads), on joint stiffness will studied by applying various levels of force to the muscle tendons and remeasuring joint stiffness. The associated hypothesis (2) is that the tension of ECU and the PQ muscles dynamically stabilize the DRUJ against volar displacement of the radius relative to the ulna. The third study will examine the efficacy of two accepted distal radio-ulnar joint reconstructions for restoring stability to the joint and for maintaining joint range of motion. Hypothesis #3 is that the Hui-Linscheid tenodesis reconstruction of the DRUJ is superior to the Fulkerson-Watson tenodesis reconstruction in allowing increased range of motion through the DRUJ, but does not provide superior constraint to displacement forces.